Why is a hot pack recommended for large fruits, like
apples, peaches and pears?

Raw packed fruits often float because they contain air.
As air escapes from the jars, liquid is carried with it.
Bringing fruits to boiling in a pan before packing them
into jars drives air out of the plant tissue. As a
result, there will be fewer problems with fruits
floating to the top of the jar. There will be less
liquid lost from the jars. There will be fewer problems
with fruits turning brown near the top of the jar. And,
more food will fit in the jars since heating shrinks
plant tissue; however, hot food should be packed fairly
loosely.